Stabilization agents for silver nanowire based transparent conductive films

ABSTRACT

Zinc salts have been found to provide anticorrosion properties when incorporated into silver nanowire containing films. Such salts may be incorporated into one of more silver nanowire containing layers or in one or more layers disposed adjacent to the silver nanowire containing layers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/778,555, filed Mar. 13, 2013, entitled STABILIZATION AGENTS FORSILVER NANOWIRE BASED TRANSPARENT CONDUCTIVE FILMS, which is herebyincorporated by reference in its entirety.

BACKGROUND

Transparent and conductive films (TCF) have been used extensively inrecent years in applications such as touch panel displays, liquidcrystal displays, electroluminescent lighting, organic light-emittingdiode devices, and photovoltaic solar cells. Indium tin oxide (ITO)based transparent conductive film has been the transparentconductor-of-choice for most applications due to its high conductivity,transparency, and relatively good stability. However, indium tin oxidebased transparent conductive films have limitations due to the high costof indium, the need for complicated and expensive vacuum depositionequipment and processes, and indium tin oxide's inherent brittleness andtendency to crack, especially when it is deposited on flexiblesubstrates.

Two of the most important parameters for measuring the properties oftransparent conductive films are total light transmittance (% T) andfilm surface electric conductivity. Higher light transmittance allowsclear picture quality for display applications, higher efficiency forlighting and solar energy conversion applications. Lower resistivity (R)is most desirable for most transparent conductive films applications inwhich power consumption can be minimized. Therefore, the higher the T/Rratio of the transparent conductive films is, the better the transparentconductive films are.

U.S. Patent Application Publication 2006/0257638A1 discloses atransparent conductive film comprising carbon nanotubes (CNT) and vinylchloride resin polymer binder.

U.S. Pat. No. 8,049,333 and U.S. Patent Application Publication2008/0286447A1 disclose a transparent conductive film in which silvernanowires are deposited onto a substrate to form a bare nanowire networkfollowed by overcoating the silver nanowire network with a polymermatrix material to form a transparent conductive film. Polymers such aspolyacrylates and carboxyl alkyl cellulose ether polymers were suggestedas useful materials for the matrix.

US Patent Application Publication 2008/0286447A1 discloses the use ofaromatic triazoles and other nitrogen containing compounds as corrosioninhibitors for silver nanowire based transparent conductors. Long chainalkylthio compounds have also been disclosed as useful corrosioninhibitors.

U.S. Patent Application Publication 2008/0292979A1 discloses atransparent conductive film comprising silver nanowires, or a mixture ofsilver nanowires and carbon nanotubes. The transparent conductivenetwork is formed either without polymer binder or in a photoimageablecomposition. The transparent conductive films were coated on both glassand polyethylene terephthalate (PET) supports.

U.S. Pat. No. 8,052,773 describes a transparent conductive film which isformed from coating of silver nanowires to form a network followed byovercoating with a layer of urethane acrylate polymer.

U.S. Patent Application Publication 2011/0024159A1 discloses use ofcorrosion inhibitors in an overcoat layer of a transparent conductivefilm.

PCT Patent Publication WO 2011/115603 discloses anticorrosion agentscomprising 1,2-diazine compounds for use in transparent conductivefilms.

US Patent Application Publication 2010/0307792A1 discloses addition ofcoordination ligands with silver nanowire aqueous dispersions to formsediments followed by separation of such sediments from the supernatantcontaining halide ions before applying such silver nanowire dispersionsin the coating and formation of TCF.

European Patent EP2251389B1 discloses a silver nanowire based inkformulation in which various aqueous silver complex ions were added intosilver nanowire based ink in a ratio of complex ion to silver nanowireof no more than 1:64 (w:w).

SUMMARY OF THE INVENTION

Zinc salts are particularly useful as anticorrosion agents for thestabilization of a network of silver nanowire-based transparentconductive films toward the undesirable reaction of such conductivefilms with corrosive agents such as hydrogen sulfide.

We have discovered that the effectiveness of zinc salts may be enhancedby their introduction in at least one coating mix for at least one layerdisposed adjacent to the at least one layer comprising silver nanowires.Such a layer might be an overcoat or topcoat layer, if disposed on theat least one layer comprising silver nanowires. Such an overcoat ortopcoat layer may, for example, be thermally cured or UV cured.Alternatively, such a layer might be a primer or undercoat layer, ifdisposed between the at least one layer comprising silver nanowires andthe transparent support. Or the zinc salt might be included in layersboth above and below the at least one layer comprising silver nanowires.In any of these cases, the zinc salt may, optionally, also be added toat least one of the layers comprising silver nanowires.

At least a first embodiment provides a transparent conductive articlecomprising a transparent support; at least one first layer disposed onthe transparent support, the at least one first layer comprising anetwork of silver nanowires dispersed within a polymer binder; and atleast one second layer disposed on the at least one first layer, the atleast one second layer comprising at least one zinc salt defined as anychemical substance which, when dissolved or put in a molten state,produces zinc cation.

In at least some embodiments, the at least one second layer may furthercomprise at least one organic acid.

In at least some such embodiments, the at least one first layer mayfurther comprise at least one zinc salt.

In yet further embodiments, the at least one first layer may comprise atleast one zinc salt, as defined above, and further comprise at least oneorganic acid.

At least a second embodiment provides a transparent conductive articlecomprising a transparent support; at least one first layer disposed onthe transparent support, the at least one first layer comprising atleast one zinc salt; and at least one second layer disposed on the atleast one first layer, the at least one second layer comprising anetwork of silver nanowires dispersed within a polymer binder.

In at least some such embodiments, the at least one first layer mayfurther comprise at least one organic acid.

In at least some such embodiments, the at least one second layer mayfurther comprise at least one zinc salt.

In yet further embodiments, the at least one second layer may compriseat least one zinc salt, and further comprise at least one organic acid.

At least a third embodiment provides a transparent conductive articlecomprising a transparent support; at least one first layer disposed onthe transparent support; at least one second layer disposed on the atleast one first layer, the at least one second layer comprising anetwork of silver nanowires dispersed within a polymer binder; at leastone third layer disposed on the at least one second layer, the at leastone third layer comprising at least one zinc salt.

In at least some embodiments, the at least one third layer may furthercomprise at least one organic acid.

In at least some such embodiments, the at least one second layer mayfurther comprise at least one zinc salt.

In yet further embodiments, the at least one second layer may compriseat least one zinc salt, as defined above, and further comprise at leastone organic acid.

At least a fourth embodiment provides methods comprising applying atleast one first coating mixture onto a transparent support to form atleast one first coated layer, the at least one first coating mixturecomprising silver nanowires and at least one polymer binder; andapplying at least one second coating mixture onto the at least one firstcoated layer to form at least one second coated layer, the at least onesecond coating mixture comprising at least one zinc salt.

In at least some such embodiments, the at least one second coatingmixture may further comprise at least one organic acid.

In at least some such embodiments, the at least one first coatingmixture may further comprise at least one zinc salt.

In yet further embodiments, the at least one first coating mixture maycomprise at least one zinc salt and at least one organic acid

At least a fifth embodiment provides methods comprising applying atleast one first coating mixture onto a transparent support to form atleast one first coated layer, the at least one first coating mixturecomprising at least one zinc salt, and applying at least one secondcoating mixture onto the at least one first coated layer, where the atleast one second coating mixture comprises silver nanowires and at leastone polymer binder.

In at least some such embodiments, the at least one first coatingmixture may further comprise at least one organic acid.

In at least some such embodiments, the at least one second coatingmixture may further comprise at least one zinc salt.

In further such embodiments, the at least one second coating mixture maycomprise at least one zinc salt and at least one organic acid.

In any of the above embodiments, the organic acid may, in some cases,comprise at least one of maleic acid (MA), tetrachlorophthalic acid(TCPA), trichloroacetic acid (TCAA), phenylphosphonic acid (PPOA),p-toluenesulfonic acid (PTSA), and phthalic acid (PA); or, in othercases, the organic acid may comprise at least one of maleic acid (MA) orp-toluenesulfonic acid (PTSA); or, in still other cases, the organicacid may comprise maleic acid. The organic acid may, for example, have apKa value from about −3.5 to about 3.0, or from about 1.85 to about 3.0.

DESCRIPTION

All publications, patents, and patent documents referred to in thisdocument are incorporated by reference in their entirety, as thoughindividually incorporated by reference.

U.S. Provisional Application No. 61/778,555, filed Mar. 13, 2013,entitled STABILIZATION AGENTS FOR SILVER NANOWIRE BASED TRANSPARENTCONDUCTIVE FILMS, is hereby incorporated by reference in its entirety.

DEFINITIONS

The terms “conductive layer” or “conductive film” refer to the networklayer comprising silver nanowires dispersed within a polymer binder.

The term “conductive” refers to electrical conductivity.

The term “article” refers to the coating of a “conductive layer” or“conductive film” on a support.

The terms “coating weight,” “coat weight,” and “coverage” aresynonymous, and are usually expressed in weight or moles per unit areasuch as g/m² or mol/m².

The term “transparent” means capable of transmitting visible lightwithout appreciable scattering or absorption.

“Haze” is wide-angle scattering that diffuses light uniformly in alldirections. It is the percentage of transmitted light that deviates fromthe incident beam by more than 2.5 degrees on the average. Haze reducescontrast and results in a milky or cloudy appearance. Materials havinglower haze percentages appear less hazy than those having higher hazepercentages.

The term “organic solvent” means “a material, liquid at use temperature,whose chemical formula comprises one or more carbon atoms.”

The term “aqueous solvent” means a material, liquid at use temperature,whose composition in a homogeneous solution comprises water in thegreatest proportion (i.e., at least 50 percent water by weight).

The term “water soluble” means the solute forms a homogenous solutionwith water, or a solvent mixture in which water is the major component.

The terms “a” or “an” refer to “at least one” of that component (forexample, the anticorrosion agents, nanowires, and polymers describedherein).

Furthermore, all publications, patents, and patent documents referred toin this document are incorporated by reference herein in their entirety,as though individually incorporated by reference.

Introduction

In order for silver based transparent conductors to have practical useit is important that these silver based transparent conductors be stablefor a long period when subjected to environmental conditions.

Any atmospheric corrosion due to the reaction of low levels of chemicalsin the air may induce undesirable chemical reactions at the metalnanowire surface, impacting the conductivity and performance of themetal nanowire based transparent conductors. It is well known thatcorrosion, or “tarnishing,” may readily occur on silver metal surfaceswhen exposed to the atmosphere. Without wishing to be bound by theory,one example of such a tarnishing mechanism is sulfidation of silversurface by reaction of hydrogen sulfide with silver:

2Ag+H₂S→Ag₂S+H₂

Because the electric conductivity of silver compounds such as silversulfide is much lower than that of silver metal, silver nanowire basedconductors can gradually lose conductivity when exposed to theatmosphere.

In contrast to bare metal wires exposed to the air, silver nanowires ina polymer matrix are more stable since the presence of the polymer slowsdown the diffusion of hydrogen sulfide (or other corrosive agents) tothe silver nanowire surface. Nevertheless, it is important to stabilizethe silver nanowire surface to prevent the sulfidation process, evenwhen the nanowires are embedded in a polymer matrix.

It would be useful to find anticorrosion agents for transparentelectrically conductive films comprising a network of silver nanowiresin polymer binder(s) that can be coated from aqueous or from organicsolvents, using common coating techniques.

Silver Nanowires

The silver nanowires are an essential component for imparting electricalconductivity to the conductive films, and to the articles prepared usingthe conductive films. The electrical conductivity of the transparentconductive film is mainly controlled by a) the conductivity of a singlenanowire, b) the number of nanowires between the terminals, and c) thenumber of connections and the contact resistivity between the nanowires.Below a certain nanowire concentration (also referred as the percolationthreshold), the conductivity between the terminals is zero, as there isno continuous current path provided because the nanowires are spaced toofar apart. Above this concentration, there is at least one current pathavailable. As more current paths are provided, the overall resistance ofthe layer will decrease. However, as more current paths are provided,the clarity (i.e., percent light transmission) of the conductive filmdecreases due to light absorption and back scattering by the nanowires.Also, as the amount of silver nanowires in the conductive filmincreases, the haze of the transparent film increases due to lightscattering by the silver nanowires. Similar effects will occur intransparent articles prepared using the conductive films.

In one embodiment, the silver nanowires have aspect ratio (length/width)of from about 20 to about 3300. In another embodiment, the silvernanowires have an aspect ratio (length/width) of from about 500 to 1000.Silver nanowires having a length of from about 5 μm to about 100 μm(micrometer) and a width of from about 10 nm to about 200 nm are useful.Silver nanowires having a width of from about 20 nm to about 100 nm anda length of from about 10 μm to about 50 μm are also particularly usefulfor construction of a transparent conductive network film.

Silver nanowires can be prepared by known methods in the art. Inparticular, silver nanowires can be synthesized through solution-phasereduction of a silver salt (e.g., silver nitrate) in the presence of apolyol (e.g., ethylene glycol or propylene glycol) and poly(vinylpyrrolidone). Large-scale production of silver nanowires of uniform sizecan be prepared according to the methods described in, e.g.,Ducamp-Sanguesa, C. et al., J. of Solid State Chemistry, (1992), 100,272-280; Sun, Y. et al., Chem. Mater. (2002), 14, 4736-4745, Sun, Y. etal., Nano Letters, (2003), 3(7), 955-960; US patent applicationpublication 2012/0063948, published Mar. 15, 2012; US patent applicationpublication 2012/0126181, published May 24, 2012; US patent applicationpublication 2012/0148436, published Jun. 14, 2012; US patent applicationpublication 2012/0207644, published Aug. 16, 2012; and US patentapplication publication 2012/0328469, published Dec. 27, 2012, each ofwhich is incorporated by reference in its entirety.

Polymer Binders

For a practical manufacturing process for transparent conductive films,it is important to have both the conductive components, such as silvernanowires, and a polymer binder in a coating dispersion. The polymerbinder solution serves a dual role, as dispersant to facilitate thedispersion of silver nanowires and as a viscosifier to stabilize thesilver nanowire coating dispersion so that the sedimentation of silvernanowires does not occur at any point during the coating process. It isalso desirable to have the silver nanowires and the polymer binder in asingle coating dispersion. This simplifies the coating process andallows for a one-pass coating, and avoids the method of first coatingbare silver nanowires to form a weak and fragile film that issubsequently over-coated with a polymer to form the transparentconductive film.

In order for a transparent conductive film to be useful in variousdevice applications, it is also important for the polymer binder of thetransparent conductive film to be optically transparent and flexible,yet have high mechanical strength, good hardness, high thermalstability, and light stability. This requires polymer binders to be usedfor transparent conductive film to have Tg (glass transitiontemperature) greater than the use temperature of the transparentconductive film.

Transparent, optically clear polymer binders are known in the art.Examples of suitable polymeric binders include, but are not limited to:polyacrylics such as polymethacrylates (e.g., poly(methylmethacrylate)), polyacrylates and polyacrylonitriles, polyvinylalcohols, polyesters (e.g., polyethylene terephthalate (PET),polybutylene terephthalate, and polyethylene naphthalate), polymers witha high degree of aromaticity such as phenolics or cresol-formaldehyde(e.g., NOVOLAC®), polystyrenes, polyvinyltoluene, polyvinylxylene,polyimides, polyamides, polyamideimides, polyetheramides, polysulfides,polysulfones, polyphenylenes, and polyphenyl ethers, polyurethane (PU),polycarbonates, epoxies polyolefins (e.g. polypropylene,polymethylpentene, and cyclic olefins), acrylonitrile-butadiene-styrenecopolymers (ABS), cellulosics, silicones and other silicon-containingpolymers (e.g. polysilsesquioxanes and polysilanes), polyvinylchloride(PVC), polyvinylacetates, polynorbornenes, synthetic rubbers (e.g. EPR,SBR, EPDM), and fluoropolymers (e.g., polyvinylidene fluoride,polytetrafluoroethylene (TFE) or polyhexafluoropropylene), copolymers offluoro-olefins and hydrocarbon olefins (e.g., LUMIFLON®), and amorphousfluorocarbon polymers or copolymers (e.g., CYTOP® by Asahi Glass Co., orTEFLON® AF by Du Pont), polyvinylbutryals, polyvinylacetals, gelatins,polysaccharides, and starches.

In certain embodiments, in order to disperse and stabilize silvernanowires in polymeric coating solution, the use of polymer bindershaving high oxygen content is advantageous. Oxygen-containing groups,such as hydroxyl group and carboxylate groups have a strong affinity forbinding to the silver nanowire surface and facilitate the dispersion andstabilization. Many oxygen-rich polymers also have good solubility inthe polar organic solvents commonly used to prepare organicsolvent-coated materials, while other oxygen-rich polymers have goodsolubility in water or the aqueous solvent mixtures commonly used toprepare aqueous solvent-coated materials.

In certain embodiments, cellulose ester polymers, such as celluloseacetate butyrate (CAB), cellulose acetate (CA), or cellulose acetatepropionate (CAP) are superior to other oxygen-rich polymer binders whenused to prepare silver nanowire based transparent conductive films thatare coated from organic solvents such as 2-butanone (methyl ethylketone, MEK), methyl iso-butyl ketone, acetone, methanol, ethanol,2-propanol, ethyl acetate, propyl acetate, butyl acetate, or mixturesthereof. Their use results in transparent conductive films in which boththe optical light transmittance and electrical conductivity of thecoated films are greatly improved. In addition, these cellulose esterpolymers have glass transition temperatures of at least 100° C. andprovide transparent, flexible films having high mechanical strength,good hardness, high thermal stability, and light stability.

The cellulose ester polymers can be present in from about 40 to about 90wt % of the dried transparent conductive films. Preferably, they arepresent in from about 60 to about 85 wt % of the dried films. In someconstructions, a mixture of a cellulosic ester polymer and one or moreadditional polymers may be used. These polymers should be compatiblewith the cellulosic polymer. By compatible is meant that a mixturecomprising at least one cellulosic ester polymer and one or moreadditional polymers forms a transparent, single phase composition whendried. The additional polymer or polymers can provide further benefitssuch as promoting adhesion to the support and improving hardness andscratch resistance. As above, total wt % of all polymers is from about40 to about 95 wt % of the dried transparent conductive films.Preferably, the total weight of all polymers is from about 60 to about85 wt % of the dried films. Polyester polymers, urethanes, andpolyacrylics are examples of additional polymers useful for blendingwith cellulosic ester polymers.

In other embodiments, water soluble polymer binders can also be used,such as polyvinyl alcohol, gelatin, polyacrylic acid, polyimides. Otherwater dispersible latex polymers can also be used such as polyacrylatesand polymethacrylates containing methyl acrylic acid units. Coating fromaqueous solutions benefits the environment and reduces the emission ofvolatile organic compounds during manufacturing.

The use of water soluble polymers, such as polyvinyl alcohol or gelatinas binders for silver nanowire based transparent conductors results insuperior transparent conductive films in which both film transmittanceand conductivity are greatly improved. Transparent conductive filmsprepared using either polyvinyl alcohol or gelatin polymer binders alsoshow excellent clarity, scratch resistance, and hardness when polymercross linkers are added to the polymer solution. Transparent conductivefilms prepared according to this invention provide transmittance of atleast 80% across entire spectrum range of about 350 nm to about 1100 nm,and surface resistivity of 500 ohm/sq or less.

The transparent conductive articles comprising silver nanowires andwater soluble polymer binders also show excellent clarity, high scratchresistance, and hardness. In addition, transparent conductive filmsprepared using these polymer binders have good adhesion to supportscomprising polyethylene terephthalate (PET), poly(methylmethacrylate),polycarbonate, and the like, when an appropriate subbing layer isapplied between the support and the conductive layer.

The water soluble polymer binders are present in from about 40 to about95 wt % of the dried transparent conductive films. Preferably, they arepresent in from about 60 to about 85 wt % of the dried films.

In some constructions, up to 50 wt % of the gelatin or polyvinyl alcoholpolymer binder can be replaced by one or more additional polymers. Thesepolymers should be compatible with the gelatin or polyvinyl alcoholpolymer binder. By compatible is meant that the all polymers form atransparent, single phase mixture when dried. The additional polymer orpolymers can provide further benefits such as promoting adhesion to thesupport and improving hardness and scratch resistance. Water solubleacrylic polymers are particularly preferred as additional polymers.Examples of such polymers are polyacrylic acid and polyacrylamides, andcopolymers thereof. As above, total wt % of all polymers is from about50 to about 95 wt % of the dried transparent conductive films.Preferably, the total weight of all polymers is from about 70 to about85 wt % of the dried films.

If desired, scratch resistance and hardness of the transparentconductive films with these polymer binders to the support can beimproved by use of crosslinking agents to crosslink the polymer binders.Isocyanates, alkoxyl silanes, and melamines are examples of typicalcrosslinking agents for cellulose ester polymers containing freehydroxyl groups. Vinyl sulfones and aldehydes are examples of typicalcrosslinking agents for gelatin binders.

Stabilization Agents

Stabilization agents are chemical compounds that, when added to thetransparent conductive film, improve the stability of the constructionwith respect to atmospheric corrosion caused by the reaction of oxygen,or one or more other chemicals in the atmosphere, with one or morecomponents in the film. This reaction results in deterioration of theelectric conductivity, optical properties, and/or physical integrity ofthe film. Stabilization agents should be colorless and odorless whenused in the transparent conductive film, and should be stable to theconditions of heat, light, and humidity in the environment wheretransparent conductive film is used.

However, in practice, many such compounds, when bound to a silvernanowire surface, will drastically reduce the electric conductivity ofthe resultant conductive film. Apparently, the insulating effect ofthese compounds prevents electron “flow” at nanowire contact points.Therefore, it is important to identify a class of compounds that willprovide anticorrosion protection to transparent conductive film withoutcausing significant reduction in conductivity and other negativeeffects. Advantageously, delaying introduction of the anticorrosionagents into the conductive nanowire network until after its formationcan minimize the destruction of conductive paths in the network.

We have found that zinc salts have anti-corrosive and stabilizingeffects when incorporated into silver nanowire containing films. Withoutwishing to be bound by theory, Applicants believe that zinc salts can beeffective sulfide ion scavengers since Zn²⁺ cations can react withsulfide ions to form insoluble ZnS salts, thereby greatly reducing theactual concentration of hydrogen sulfide available to react with silvernanowires after diffusing from air into AgTCF layer.

In some embodiments, the zinc salts may comprise zinc nitrate hydrate(Zn(NO₃)₂.xH₂O). Such a hydrate of zinc nitrate may be provided as oneof its particular hydrates or as a mixture of one or more of itshydrates. Zinc nitrate is known to have hydrates for integral values ofx from 2 to 6.

In at least some embodiments, anticorrosion compounds may comprise atleast one zinc salt and further comprise at least one organic acid. Somenon-limiting examples of organic acids are maleic acid (MA),tetrachlorophthalic acid (TCPA), trichloroacetic acid (TCAA),phenylphosphonic acid (PPOA), p-toluenesulfonic acid (PTSA), andphthalic acid (PA). In at least some embodiments, the organic acidscomprise pKa values from about −3.5 to about 3.0.

In yet further embodiments the anticorrosion compound may comprise zincnitrate hydrate (Zn(NO₃)₂.xH₂O) and at least one of the following sixorganic acids:

In at least some embodiments, the organic acids have pKa values fromabout −3.5 to about 3.0, or from about 1.85 to about 3.0.

In some cases, the organic acid comprises at least one of maleic acid orpara-toluenesulfonic acid. In still other cases, the organic acidcomprises maleic acid.

Coating of the Conductive Films

An organic solvent-based coating formulation for the transparent silvernanowire films can be prepared by mixing the various components with oneor more polymer binders in a suitable organic solvent system thatusually includes one or more solvents such as toluene, 2-butanone(methyl ethyl ketone, MEK), methyl iso-butyl ketone, acetone, methanol,ethanol, 2-propanol, ethyl acetate, propyl acetate, butyl acetate, ethyllactate, tetrahydrofuran, or mixtures thereof. An aqueous-based coatingformulation for the transparent silver nanowire films can be prepared bymixing the various components with one or more polymer binders in wateror in a mixture of water with a water miscible solvent such as acetone,acetonitrile, methanol, ethanol, 2-propanol, or tetrahydrofuran, ormixtures thereof. Transparent films containing silver nanowires can beprepared by coating the formulations using various coating proceduressuch as wire wound rod coating, dip coating, knife, or blade coating,curtain coating, slide coating, slot-die coating, roll coating, orgravure coating. Surfactants and other coating aids can be incorporatedinto the coating formulation.

In one embodiment the coating weight of the silver nanowires is fromabout 10 mg/m² to about 500 mg/m². In another embodiment the coatingweight of silver nanowires is from about 20 m g/m² to about 200 mg/m².In a further embodiment, the coating weight of silver nanowires is fromabout 30 mg/m² to about 120 mg/m². A useful coating dry thickness of thetransparent conductive coating is from about 0.05 μm to about 2.0 μm,and preferably from about 0.1 μm to about 0.5 μm.

Upon coating and drying, the transparent conductive film should have asurface resistivity of less than 1,000 ohms/sq and preferably less than500 ohm/sq.

Upon coating, and drying, the transparent conductive film should have ashigh a % transmittance as possible. A transmittance of at least 70% isuseful. A transmittance of at least 80% and even at least 90% are evenmore useful.

Particularly useful are films with a transmittance of at least 70% and asurface resistivity of less than 500 ohm/sq.

Such transparent conductive films provide transmittance of at least 80%across entire spectrum range of from about 350 nm to about 1100 nm, andsurface resistivity of less than 500 ohm/sq.

Transparent Support

In one embodiment, the conductive materials are coated onto a support.The support may be rigid or flexible.

Suitable rigid substrates include, for example, glass, polycarbonates,acrylics, and the like.

When the conductive materials are coated onto a flexible support, thesupport is preferably a flexible, transparent polymeric film that hasany desired thickness and is composed of one or more polymericmaterials. The support is required to exhibit dimensional stabilityduring coating and drying of the conductive layer and to have suitableadhesive properties with overlying layers. Useful polymeric materialsfor making such supports include polyesters [such as poly(ethyleneterephthalate) (PET) and poly(ethylene naphthalate) (PEN)], celluloseacetates and other cellulose esters, polyvinyl acetal, polyolefins,polycarbonates, and polystyrenes. Preferred supports are composed ofpolymers having good heat stability, such as polyesters andpolycarbonates. Support materials may also be treated or annealed toreduce shrinkage and promote dimensional stability. Transparentmultilayer supports can also be used.

Coating of the Conductive Films onto a Support

Transparent conductive articles can be prepared by coating theformulations described above onto a transparent support using variouscoating procedures such as wire wound rod coating, dip coating, knifecoating, curtain coating, slide coating, slot-die coating, roll coating,gravure coating, or extrusion coating.

Alternatively, transparent conductive articles can be prepared bylaminating the transparent conductive films prepared as described aboveonto a transparent support.

In some embodiments, a “carrier” layer formulation comprising asingle-phase mixture of two or more polymers may be applied directlyonto the support and thereby located between the support and the silvernanowire layer. The carrier layer serves to promote adhesion of thesupport to the transparent polymer layer containing the silvernanowires. The carrier layer formulation can be sequentially orsimultaneously applied with application of the transparent conductivesilver nanowire layer formulation. It is preferred that all coating beapplied simultaneously onto the support. Carrier layers are oftenreferred to as “adhesion promoting layers,” “interlayers,” or“intermediate layers.”

As noted above, in one embodiment the coating weight of the silvernanowires is from about 20 mg/m² to about 500 mg/m². In otherembodiments, coating weight of silver nanowires is from about 10 m g/m²to about 200 mg/m². Embodiments wherein the silver nanowires are coatedat from about 10 mg/m² to about 120 mg/m² are also contemplated.

Upon coating and drying, the transparent conductive article should havea surface resistivity of less than 1,000 ohms/sq and preferably lessthan 500 ohm/sq.

Similarly, upon coating and drying on a transparent support, thetransparent conductive article should have as high an opticaltransmittance as possible. A transmittance of at least 70% is useful. Atransmittance of at least 80% and even at least 90% are even moreuseful.

Particularly preferred are articles with a transmittance of at least 80%and a surface resistivity of less than 500 ohm/sq.

The following examples are provided to illustrate the practice of thepresent invention and the invention is not meant to be limited thereby.

ENUMERATED EMBODIMENTS

U.S. Provisional Application No. 61/778,555, filed Mar. 13, 2013,entitled STABILIZATION AGENTS FOR SILVER NANOWIRE BASED TRANSPARENTCONDUCTIVE FILMS, which is hereby incorporated by reference in itsentirety, disclosed the following 58 non-limiting enumeratedembodiments:

A. A transparent conductive article comprising:

a transparent support;

at least one first layer disposed on the transparent support, the atleast one first layer comprising a network of silver nanowires dispersedwithin at least one polymer binder, and;

at least one second layer disposed on the at least one first layer, theat least one second layer comprising at least one zinc salt.

B. The transparent conductive article of embodiment A, wherein the atleast one zinc salt comprises zinc nitrate hydrate.C. The transparent conductive article of embodiment A, wherein the atleast one second layer further comprises at least one organic acid.D. The transparent conductive article of embodiment C, wherein the atleast one organic acid comprises at least one of maleic acid,tetrachlorophthalic acid, trichloroacetic acid, phenylphosphonic acid,p-toluenesulfonic acid, or phthalic acid.E. The transparent conductive article according to embodiment C, whereinthe at least one organic acid comprises at least one of maleic acid orp-toluenesulfonic acid.F. The transparent conductive article according to embodiment C, whereinthe at least one organic acid comprises maleic acid.G. The transparent conductive article of embodiment A, wherein thetransparent support is a flexible transparent polymer film.H. The transparent conductive article of embodiment A, wherein thesilver nanowires are present in an amount sufficient to provide asurface resistivity of less than about 1000 ohm/sq.J. The transparent conductive article of embodiment A, wherein thesilver nanowires have an aspect ratio of from about 20 to about 3300.K. The transparent conductive article of embodiment A, wherein thesilver nanowires are present in an amount of from about 10 mg/m² toabout 500 mg/m².L. The transparent conductive article of embodiment A, having atransmittance of at least about 80% across entire spectrum range of fromabout 350 nm to about 1100 nm and a surface resistivity of 500 ohm/sq orless.M. The transparent conductive article of embodiment A, wherein the atleast one polymer binder comprises at least one water soluble polymer.N. The transparent conductive article of embodiment M, wherein the atleast one water soluble polymer comprises gelatin, polyvinyl alcohol, ormixtures thereof.P. The transparent conductive article of embodiment N, wherein the atleast one polymer binder further comprises up to about 50 wt % of one ormore additional water soluble polymers.Q. The transparent conductive article of embodiment P, wherein one ormore of the additional water soluble polymers is a polyacrylic polymer.R. The transparent conductive article of embodiment A, wherein the atleast one polymer binder comprises an organic solvent soluble polymer.S. The transparent conductive article of embodiment R, wherein theorganic solvent soluble polymer binder comprises at least one celluloseester polymer.T. The transparent conductive article of embodiment R, wherein theorganic solvent soluble polymer binder comprises cellulose acetate,cellulose acetate butyrate, or cellulose acetate propionate, or mixturesthereof.U. The transparent conductive article of embodiment S, wherein the atleast one cellulose ester polymer has a glass transition temperature ofat least about 100° C.V. The transparent conductive article of embodiment R, wherein the atleast one polymer binder further comprises up to 50 wt % of one or moreadditional organic solvent soluble polymers.W. The transparent conductive article of embodiment V, wherein one ormore of the additional organic solvent soluble polymers is a polyesterpolymer.X. A transparent conductive article comprising:

a transparent support;

at least one first layer disposed on the transparent support, the atleast one first layer comprising a network of silver nanowires and apolymer binder, and at least one zinc salt; and,

at least one second layer consisting of a transparent polymer.

Y. The transparent conductive article of embodiment X, wherein the saidat least one zinc salt comprises zinc nitrate hydrate.Z. The transparent conductive article of embodiment X, wherein the atleast one first layer further comprises at least one organic acid.AA. The transparent conductive article of embodiment Z, wherein the atleast one organic acid comprises at least one of maleic acid,tetrachlorophthalic acid, trichloroacetic acid, phenylphosphonic acid,p-toluenesulfonic acid, or phthalic acid.AB. The transparent conductive article according to embodiment Z,wherein the at least one organic acid comprises at least one of maleicacid or p-toluenesulfonic acid.AC. The transparent conductive article according to embodiment Z,wherein the organic acid comprises maleic acid.AD. The transparent conductive article of embodiment X, wherein thetransparent support is a flexible transparent polymer film.AE. The transparent conductive article of embodiment X, wherein thesilver nanowires are present in an amount sufficient to provide asurface resistivity of less than about 1000 ohm/sq.AF. The transparent conductive article of embodiment X, wherein thesilver nanowires have an aspect ratio of from about 20 to about 3300.AG. The transparent conductive article of embodiment X, wherein thesilver nanowires are present in an amount of from about 10 mg/m² toabout 500 mg/m².AH. The transparent conductive article of embodiment X, having atransmittance of at least about 80% across entire spectrum range of fromabout 350 nm to about 1100 nm and a surface resistivity of 500 ohm/sq orless.AJ. The transparent conductive article of embodiment X, wherein the atleast one polymer binder comprises at least one water soluble polymer.AK. The transparent conductive article of embodiment AJ, wherein the atleast one water soluble polymer comprises gelatin, polyvinyl alcohol, ormixtures thereof.AL. The transparent conductive article of embodiment AK, wherein the atleast one polymer binder further comprises up to about 50 wt % of one ormore additional water soluble polymers.AM. The transparent conductive article of embodiment AL, wherein one ormore of the additional water soluble polymers is a polyacrylic polymer.AN. The transparent conductive article of embodiment X, wherein the atleast one polymer binder comprises an organic solvent soluble polymer.AP. The transparent conductive article of embodiment AN, wherein theorganic solvent soluble polymer binder comprises at least one celluloseester polymer.AQ. The transparent conductive article of embodiment AP, wherein the atleast one cellulose ester polymer has a glass transition temperature ofat least about 100° C.AR. The transparent conductive article of embodiment AN, wherein theorganic solvent soluble polymer binder comprises cellulose acetate,cellulose acetate butyrate, or cellulose acetate propionate, or mixturesthereof.AS. The transparent conductive article of embodiment X, wherein the atleast one polymer binder further comprises up to 50 wt % of one or moreadditional organic solvent soluble polymers.AT. The transparent conductive article of embodiment AS, wherein one ormore of the additional organic solvent soluble polymers is a polyesterpolymer.AU. A method comprising:

applying at least one first coating mixture onto a transparent supportto form at least one first coated layer, the at least one first coatingmixture comprising silver nanowires and at least one polymer binder;and,

applying at least one second coating mixture onto the at least one firstcoated layer to form at least one second coated layer, the at least onesecond coating mixture comprising at least one zinc salt.

AV. The method according to embodiment AU, wherein the at least one zincsalt comprises zinc nitrate hydrate.AW. The method according to embodiment AU, wherein the at least onesecond coating mixture further comprises at least one organic acid.AX. The transparent conductive article of embodiment AW, wherein the atleast one organic acid comprises at least one of maleic acid,tetrachlorophthalic acid, trichloroacetic acid, phenylphosphonic acid,p-toluenesulfonic acid, or phthalic acid.AY. The method according to embodiment AW, wherein the at least oneorganic acid comprises at least one of maleic acid or p-toluenesulfonicacid.AZ. The method according to embodiment AW, wherein the at least oneorganic acid comprises maleic acid.BA. The method according to embodiment AU, wherein applying the at leastone first coating mixture and applying the at least one second coatingmixture occur simultaneously.BB. The method according to embodiment AU, further comprising drying theat least one first layer or the at least one second layer or both.BC. A method comprising:

applying at least one first coating mixture onto a transparent supportto form at least one first coated layer, the at least one first coatingmixture comprising at least one zinc salt, silver nanowires, and atleast one polymer binder; and,

applying at least one second coating mixture onto the at least one firstcoated layer.

BD. The method according to embodiment BC, wherein the at least one zincsalt comprises zinc nitrate hydrate.BE. The method according to embodiment BC, wherein the at least onefirst coating mixture further comprises at least one organic acid.BF. The transparent conductive article of embodiment BE, wherein the atleast one organic acid comprises at least one of maleic acid,tetrachlorophthalic acid, trichloroacetic acid, phenylphosphonic acid,p-toluenesulfonic acid, or phthalic acid.BG. The method according to embodiment BE, wherein the at least oneorganic acid comprises at least one of maleic acid or p-toluenesulfonicacid.BH. The method according to embodiment BE, wherein the at least oneorganic acid comprises maleic acid.BJ. The method according to embodiment BC, wherein applying the at leastone first coating mixture and applying the at least one second coatingmixture occur simultaneously.BK. The method according to embodiment BC, further comprising drying theat least one first layer or the at least one second layer or both.

EXAMPLES Materials and Methods

All materials used in the following examples are readily available fromstandard commercial sources, such as Sigma-Aldrich Chemical Co. LLC(Saint Louis) unless otherwise specified. All percentages are by weightunless otherwise indicated. The following additional materials andmethods were used.

Zinc nitrate hydrate, Zn(NO₃)₂.xH₂O (Mallinckrodt).

Maleic acid (MA), available from Sigma-Aldrich, having the structure:

p-toluenesulfonic acid (PTSA), available from Fisher/Univar(Martinsville, Va.), having the structure:

CAB 381-20 is a cellulose acetate butyrate resin available from EastmanChemical Co. (Kingsport, Tenn.). It has a glass transition temperatureof 141° C.

CAB 553-0.4 is a cellulose acetate butyrate resin available from EastmanChemical Co. (Kingsport, Tenn.). It has a glass transition temperatureof 136° C.

CYMEL® 303 crosslinker is hexamethoxymethylmelamine, available fromCytec Industries (West Paterson, N.J.).

ESTAR® polyester substrate is available from Eastman Kodak (Rochester,N.Y.).

SLIP-AYD® FS 444 (polysiloxane in dipropylene glycol, Elementis) is aliquid additive for increasing surface slip and mar resistance of waterborne and polar solvent borne coatings.

Silver nanowires were prepared according to procedures described in U.S.patent application Ser. 14/043,966, filed Oct. 2, 2013, which is herebyincorporated by reference in its entirety. The typical silver nanowireshave diameters ranging from 38 nm to 44 nm and range in length from 17to 25 μm.

Example 1 Preparation of Silver Nanowire Coating Dispersion

A CAB polymer premix solution was prepared by mixing 15 parts by weightof CAB 381-20 (cellulose acetate butyrate polymer, Eastman Chemical)with 85 parts by weight of n-propyl acetate (Oxea). The resulting CABpolymer premix solution was filtered prior to use.

15.00 parts by weight of the CAB polymer premix solution was combinedwith 10.00 parts by weight ethyl lactate (>99.8% purity), 40.55 parts byweight of a 1.85% solids dispersion of silver nanowires in isopropanol,and 34.44 parts by weight of n-propyl acetate (Oxea) to form a silvernanowire coating dispersion at 3.00% solids.

The finished silver nanowire coating dispersion was coated on a labproofer with a 380 LPI plate onto 5 mil ESTAR LS polyester support, anddried at 275° F. for 2 minutes.

Preparation of Topcoat Solutions

A CAB polymer premix solution was prepared by mixing 15 parts by weightof CAB 553-0.4 (cellulose acetate butyrate polymer, Eastman Chemical)into 42.50 parts by weight of denatured ethanol and 42.50 parts byweight methanol (>99% purity). The resulting CAB polymer premix solutionwas filtered prior to use.

A topcoat masterbatch solution was prepared by adding to 5000 parts byweight of the CAB polymer premix solution 7450 parts by weight denaturedethanol, 4500 parts by weight of 33 wt % CYMEL 303(hexamethoxymethylmelamine, Cytec) in denatured ethanol, 150 parts byweight of 10 wt % SLIP-AYD FS-444 (polysiloxane in dipropylene glycolether, Elementis) in denatured ethanol, and 1940 parts by weight ofn-butanol (>98% purity). The topcoat masterbatch solution had 12.0%solids.

Topcoat solutions were prepared by adding various loadings ofZn(NO₃)₂.xH₂O, p-toluenesulfonic acid (PTSA), and maleic acid (MA) toaliquots of the masterbatch solution, as shown in Table I.

Preparation of the Coated Films

The topcoat solutions were overcoated on the silver nanowire thinconducting film (AgTCF) described above with a 450 LPI plate. Thecoatings were then dried in an oven at 275° F. for 3 minutes.

Desktop TCF Stability Test

Surface resistivity was measured for the coatings immediately aftercoating (initial values) with either a RCHEK RC3175 4-point resistivityor a Delcom 707 non-contact conductance monitor. These TCF samples werethen placed on lab desktop under 1500-2000 LUX fluorescence light withthe TCF side towards the light for 1, 2, and 4 months. After the testperiod, the TCF samples were then checked again to record the change infilm resistivity.

The stability testing results showed the desktop stability was improvedupon addition of either zinc nitrate hydrate (Zn(NO₃)₂.xH₂O), acombination of zinc nitrate hydrate and p-toluenesulfonic acid (PTSA),or a combination of zinc nitrate hydrate and maleic acid (MA). Sample1-4, having a topcoat layer comprising zinc nitrate hydrate and maleicacid, showed the best results of those examined.

The invention has been described in detail with particular reference toa presently preferred embodiment, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention. The presently disclosed embodiments are thereforeconsidered in all respects to be illustrative and not restrictive. Thescope of the invention is indicated by the appended claims, and allchanges that come within the meaning and range of equivalents thereofare intended to be embraced therein.

TABLE I Surface Surface Surface Resistivity Resistivity Resistivity PTSAInitial Total Change Change Change Zn(NO₃)₂•xH₂0 in MA in Surface lightDesktop Desktop Desktop in topcoat topcoat topcoat Resistivity trans.Haze t = 1 month t = 2 months t = 4 months Sample # (wt %) (wt %) (wt %)(ohms/sq) (% T) (%) (% change) (% change) (% change) Com-1-1 none nonenone 53 90.1 1.88 +42 +136 +323 1-1 0.73 none none 57 89.6 1.66 +17 +53+332 1-2 1.36 none none 58 89.6 1.62 +15 +48 +180 1-3 1.36 0.13 none 5889.3 1.57 +7 +44 +168 1-4 1.36 none 0.57 65 89.4 1.67 −13 −13 −12

What is claimed:
 1. A transparent conductive article comprising: atransparent support; at least one first layer disposed on thetransparent support, the at least one first layer comprising a networkof silver nanowires dispersed within at least one polymer binder; and atleast one zinc salt.
 2. The transparent conductive article according toclaim 1, wherein the at least one zinc salt comprises zinc nitratehydrate.
 3. The transparent conductive article according to claim 1,wherein the at least one first layer further comprises the at least onezinc salt.
 4. The transparent conductive article according to claim 3,wherein the at least one first layer further comprises at least oneorganic acid.
 5. The transparent conductive article according to claim4, wherein the at least one organic acid comprises at least one ofmaleic acid, tetrachlorophthalic acid, trichloroacetic acid,phenylphosphonic acid, p-toluenesulfonic acid, or phthalic acid.
 6. Thetransparent conductive article according to claim 1, further comprisingat least one second layer disposed adjacent to the at least one firstlayer, the at least one second layer comprising the at least one zincsalt.
 7. The transparent conductive article according to claim 6,wherein that at least one second layer is disposed on the at least onefirst layer.
 8. The transparent conductive article according to claim 6,wherein the at least one second layer is disposed between the at leastone transparent substrate and the at least one first layer.
 9. Thetransparent conductive article according to claim 6, wherein the atleast one first layer further comprises at least one organic acid. 10.The transparent conductive article according to claim 9, wherein the atleast one organic acid comprises at least one of maleic acid,tetrachlorophthalic acid, trichloroacetic acid, phenylphosphonic acid,p-toluenesulfonic acid, or phthalic acid.
 11. The transparent conductivearticle according to claim 9, wherein the at least one organic acidcomprises maleic acid or p-toluenesulfonic acid.
 12. A methodcomprising: applying at least one first coating mixture onto atransparent support to form at least one first coated layer, the atleast one first coating mixture comprising silver nanowires and at leastone polymer binder; and, applying at least one second coating mixtureonto the at least one first coated layer to form at least one secondcoated layer, the at least one second coating mixture comprising atleast one zinc salt.